Polymer composites containing carbon nanotubes and methods related thereto

ABSTRACT

Polymer composites containing carbon nanotubes often exhibit high glass transition temperatures, which can complicate their use in additive manufacturing processes. Extruded filaments containing carbon nanotubes and residual solvent can have desirably lowered glass transition temperatures. Extruded filaments can contain a polymer as a continuous phase, a nanomaterial such as carbon nanotubes homogeneously mixed throughout the continuous phase, and above 0% to about 15% solvent by weight. Methods for making extruded filaments can include producing a solvated composite by dissolving a polymer and a nanomaterial in a solvent, producing a partially desolvated composite by reducing a solvent content of the solvated composite to a range of about 10% to about 30% by weight, forming particles of the partially desolvated composite, supplying the particles to an extruder, and extruding a filament having the polymer as a continuous phase and the nanomaterial homogeneously mixed throughout the continuous phase, which also contains residual solvent.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD

The present disclosure generally relates to nanomaterials and, morespecifically, to polymer composites containing carbon nanotubes.

BACKGROUND

Three-dimensional (3-D) printing, also known as additive manufacturing,is a rapidly growing technology area that operates by depositing smalldroplets or streams of a melted or softened printing material in precisedeposition locations under the control of a computer. Deposition of theprinting material results in gradual, layer-by-layer buildup of aprinted object, which can be in any number of complex shapes. Atpresent, additive manufacturing processes are largely used for rapidprototyping purposes, but there is significant interest in extendingthese techniques to mass manufacturing. While printed prototypes neednot necessarily be entirely functional or mechanically robust, massmanufactured objects need to be. At present, additive manufacturingprocesses do not yet have effective solutions for these and severalother issues, as discussed hereinafter.

Polymers are among the more commonly used printing materials in additivemanufacturing processes, although non-polymeric printing materials canbe used in some instances. One problematic feature of polymer-basedprinting materials is that most polymers are not electricallyconductive. When electrical conductivity of a finished object isnecessary, the poor electrical conductivity of polymers can limit thebreadth of printed objects that can be suitably produced using additivemanufacturing techniques. Polymers can also lack sufficient mechanicalstrength and thermal conductivity for some high-performanceapplications.

There has been ongoing interest in incorporating carbon nanotubes andother nanomaterials within polymer composites due to the ability ofthese nanomaterials to convey electrical conductivity to an otherwisenon-conductive polymer matrix, as well as to improve mechanical strengthand other properties. Although some success has been realized inincorporating carbon nanotubes into polymer matrices, the carbonnanotubes are often not effectively dispersed from one another andcompositional heterogeneity frequently results. Among other undesirablefactors, compositional heterogeneity can lead to structural weak pointsin the composite.

In general, the incorporation of carbon nanotubes into a polymersignificantly increases the glass transition temperature of theresulting polymer composite. The increased glass transition temperaturecan approach or exceed the decomposition temperature of the polymeritself, which can make carbon nanotube polymer composites difficult orimpossible to use in conventional additive manufacturing processes. Inaddition, the compositional heterogeneity of many carbon nanotubepolymer composites remains an ongoing concern for their potential use informing printed objects, especially for high-performance applications.

In view of the foregoing, improved materials for use in additivemanufacturing processes would be highly desirable in the art. Thepresent disclosure satisfies the foregoing need and provides relatedadvantages as well.

SUMMARY

In some embodiments, the present disclosure describes methods forforming a polymer composite and extruded filament. The methods include:dissolving a polymer and a nanomaterial in a solvent, thereby producinga solvated composite; reducing a solvent content of the solvatedcomposite to within a range of about 10% to about 30% by weight, therebyproducing a partially desolvated composite; forming first particles ofthe partially desolvated composite; supplying the first particles to anextruder; and extruding a first filament containing the polymer as acontinuous phase and the nanomaterial homogenously mixed throughout thecontinuous phase. At least a portion of the solvent remains in the firstfilament.

In some embodiments, the present disclosure describes extruded filamentsand foams formed from a polymer composite. The extruded filamentsinclude a polymer as a continuous phase, a nanomaterial homogeneouslymixed throughout the continuous phase, and above 0% to about 15% solventby weight. Printed objects and additive manufacturing processesemploying the extruded filaments are also disclosed herein.

The foregoing has outlined rather broadly the features of the presentdisclosure in order that the detailed description that follows can bebetter understood. Additional features and advantages of the disclosurewill be described hereinafter. These and other advantages and featureswill become more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1 shows an illustrative SEM image of the extruded filament producedin Example 3; and

FIG. 2 shows an illustrative TGA profile of the extruded filamentproduced in Example 3.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to methods for producing anextruded filament defined by a nanomaterial polymer composite, such as acarbon nanotube polymer composite. The present disclosure is alsodirected, in part, to extruded filaments and foams defined by ananomaterial polymer composite, such as a carbon nanotube polymercomposite. The present disclosure is also directed, in part, to printedobjects and additive manufacturing processes employing an extrudedfilament defined by a carbon nanotube polymer composite.

Carbon nanotubes have been widely proposed for use in a number ofapplications in order to take advantage of their beneficial combinationof chemical, mechanical, electrical, and thermal properties. There are anumber of difficulties associated with expressing these desirableproperties in an end product, however. For example, direct incorporationof carbon nanotubes into a polymer matrix via melt blending can resultin poor dispersion of the carbon nanotubes from themselves (i.e., viaexfoliation or debundling) and incomplete mixing of the carbon nanotubesthroughout the composite. Solvent-based blending techniques, incontrast, can be complicated by the limited solubility ofnon-functionalized carbon nanotubes in most solvents, in addition to lowpolymer solubility values in many instances. Although varioussolubility-improving carbon nanotube functionalization techniques arenow available, they frequently diminish the extremely desirableelectrical conductivity properties of non-functionalized carbonnanotubes.

Incorporation of carbon nanotubes into a polymer frequently increasesthe glass transition temperature of the resulting carbon nanotubepolymer composite. For some applications, the increased glass transitiontemperature can be desirable. However, in applications where subsequentreflow of the polymer composite is needed, such as during 3-D printingand other additive manufacturing processes, the exceedingly high glasstransition temperature of high-performance polymers containing carbonnanotubes can be problematic. Specifically, the polymer itself candegrade below the effective glass transition temperature, or the glasstransition temperature can simply be so high that the carbon nanotubepolymer composite is incompatible with print heads used in conventionaladditive manufacturing processes. In addition, the low solubility ofcarbon nanotubes has conventionally precluded their use in additivemanufacturing processes.

The present inventors made a number of concurrent discoveries that canfacilitate the production of polymer composites containing carbonnanotubes and/or other nanomaterials, as discussed hereinafter. Althoughthe present disclosure is largely directed to carbon nanotube polymercomposites, particularly composite forms such as foams and extrudedfilaments, it is to be recognized that other nanomaterials (e.g.,nanoparticles, nanowires, graphene, carbon black and the like) can beutilized similarly using some or all of the disclosure herein. Thesealternative nanomaterials can be similarly difficult to incorporatehomogeneously within a polymer composite.

More specifically, the inventors discovered solution-based processesthat can be used to dissolve both carbon nanotubes and/or anothernanomaterial and a polymer to produce a solvated polymer composite. Thesolvent content of the solvated polymer composite can then be graduallyreduced through various processing operations to produce an extrudedfilament of a polymer composite, which is defined by a continuous phaseof the polymer and carbon nanotubes and/or another nanomaterialhomogeneously mixed throughout the continuous phase. The inventors foundthat having residual solvent throughout the various processingoperations advantageously facilitates extrusion by lowering the glasstransition temperature of the polymer composite. The amount of residualsolvent can, in some instances, be dictated by the intended end use ofthe polymer composite.

The inventors further found that by adjusting the amount of residualsolvent in an extruded filament of a polymer composite, the glasstransition temperature can remain low enough to conduct additivemanufacturing processes using conventional printing equipment while notcompromising the integrity of the resulting printed object due tooutgassing and void formation. The terms “part,” “tool” and “article”may be used synonymously herein with the terms “object” and “printedobject.” Desirably, an extruded filament of the present disclosure cancontain a non-zero amount of residual solvent up to about 5% by weightof the extruded filament, or more particularly up to about 3% by weightof the extruded filament. Higher amounts of residual solvent can also beacceptable in certain instances.

After printing, further solvent removal occurs and the glass transitiontemperature increases. The subsequent increase in glass transitiontemperature upon printing can also be advantageous when considering theoperating conditions for manufactured parts. The typical thermoplasticsused in fused deposition modeling (FDM) manufacturing are, bydefinition, able to be re-melted at the glass transition temperature,which limits the thermal envelope one might expose such articles to upondeployment. This can be especially true when the article is load-bearingin nature, for example, and the softening of the polymer during heatexposure might result in mechanical failure. The drastic increase inglass transition temperature of the polymer composites of the presentdisclosure, in contrast, can extend the viable thermal envelope of sucharticles. In terms of polymer classification, the presently describedpolymer composites behave more like thermosets after an article has beenformed, which can be much more desirable from an operational andmanufacturing standpoint. As such, vastly improved mechanical strengthcan result by virtue of the present disclosure.

To facilitate the solution-based composite manufacturing processesdiscovered by the inventors, a relatively high-boiling solvent havinggood dissolution properties for both the polymer and the carbonnanotubes can be chosen. The inventors found that o-dichlorobenzene canbe particularly desirable for this purpose due to its ability todissolve both polymers and carbon nanotubes at high concentrations. Inaddition, the relatively high boiling point of this solvent allows it tobe retained in an extruded filament after processing is complete. Othersolvents can also suitable for use, as discussed hereinbelow.

Moreover, the inventors further discovered techniques whereby carbonnanotubes can be dispersed from one another prior to being combined withthe polymer, which can promote uniform mixing of the carbon nanotubes ina polymer composite. A modified chlorosulfonic acid dispersion processcan be used for this purpose. Desirably, the dispersion techniquesidentified by the inventors allow the carbon nanotubes to remain in anon-functionalized state following their dispersion from one another,thus maintaining their high electrical conductivity values onceincorporated within a polymer composite. Further, no surfactants becomeassociated with the carbon nanotubes in the dispersion processesidentified by the inventors. In addition to facilitating the formationof carbon nanotube polymer composites, the carbon nanotube dispersiontechniques described herein can also be independently practiced toproduce dispersed carbon nanotubes for other applications.

In addition to their ability to convey electrical conductivity to aprinted object, carbon nanotubes can also improve a printed object'smechanical properties. The ability to homogeneously mix carbon nanotubeswith polymers in the present disclosure can allow mechanical propertiesto be expressed more strongly than when the carbon nanotubes arenon-uniformly mixed. As such, the present disclosure is alsoadvantageous from the standpoint of allowing improved mechanical andelectrical properties to be realized simultaneously in a printed object.Furthermore, the carbon nanotubes can significantly improve the thermalconductivity of polymers that are otherwise poorly thermally conductive.

Accordingly, the present disclosure describes carbon nanomaterialpolymer composites containing a non-zero amount of residual solvent,more specifically a carbon nanotube polymer composite in certainembodiments. The carbon nanomaterial polymer composites can be in anydesired form, although extruded filaments and foams can be particularlydesirable forms for utilization in downstream processes, such asadditive manufacturing processes. Extruded filaments can be formedaccording to the processes described herein. Foaming techniques areunderstood by persons having ordinary skill in the art, and the presenceof the carbon nanomaterial does not significantly impact the foamingprocess.

In various embodiments, extruded filaments of the present disclosure caninclude a polymer as a continuous phase, a nanomaterial homogeneouslymixed throughout the continuous phase, and above 0% to about 15% solventby weight. Depending on the solvent content, the extruded filaments canbe in solid or semi-solid form. In more particular embodiments, thenanomaterial can be carbon nanotubes or a mixture of carbon nanotubeswith another nanomaterial, wherein the extruded filaments contain carbonnanotubes homogeneously mixed throughout the continuous phase defined bythe polymer.

Polymer composites containing carbon nanotubes can be electricallyconductive if the carbon nanotubes are initially electrically conductiveand are handled properly during formation of the polymer composites,particularly when forming dispersed carbon nanotubes. Illustrativeprocesses for producing dispersed carbon nanotubes that maintainsubstantially the same electrical conductivity values as pristine carbonnanotubes are discussed hereinbelow. The dispersion processes of thepresent disclosure advantageously provide for the carbon nanotubes toremain non-functionalized during and following the dispersion process,as discussed hereinbelow. The dispersion processes disclosed herein canbe employed in conjunction with the filament-forming processes of thepresent disclosure, or the dispersion processes can be practicedindependently in producing dispersed carbon nanotubes for otherpurposes. Further advantageously, the dispersion processes disclosedherein can produce dispersed carbon nanotubes that are surfactant free,such that no surfactants become incorporated in the polymer compositesor extruded filaments of the present disclosure. Improved properties ofthe polymer composites and extruded filaments can also be realized bykeeping their continuous phases free of surfactants. For example,surfactants can at least partially degrade otherwise favorablemechanical and electrical properties.

Accordingly, in some embodiments, the polymer composites, extrudedfilaments and foams disclosed herein can be electrically conductive,provided that the carbon nanotubes are present above a percolationthreshold concentration in the continuous phase. In more particularembodiments, extruded filaments of the present disclosure can containabout 0.0001% to about 10% carbon nanotubes by weight, or about 0.001 toabout 8% carbon nanotubes by weight, or about 0.01% to about 5% carbonnanotubes by weight. Higher weight percentages of carbon nanotubes canresult in increased electrical conductivity values, provided that thecarbon nanotubes continue to remain capable of being mixed homogeneouslyin the continuous phase defined by the polymer.

In more particular embodiments, the carbon nanotubes incorporated in thepolymer composites and extruded filaments of the present disclosure canbe single-walled carbon nanotubes. In still more particular embodiments,at least a portion of the carbon nanotubes incorporated in the polymercomposites and extruded filaments can be single-walled carbon nanotubesthat have metallic chiralities. As used herein, the term “carbonnanotube chirality” refers to a double index (n,m) describing aparticular carbon nanotube, where n and m are integers that describe thecut and wrapping of hexagonal graphite when formed into a tubularstructure. Such designation of a carbon nanotube's chirality will befamiliar to one having ordinary skill in the art. The term“semiconducting carbon nanotube” refers to a carbon nanotube that isdefined by the relationship |m−n|=3k+1, where k is an integer, and theterm “metallic carbon nanotube” refers to a carbon nanotube that isdefined by the relationship |m−n|=3k, where k is an integer. Carbonnanotubes may be further characterized as being “zig-zag chirality” or“armchair chirality” based upon their chiral indices. For example,metallic carbon nanotubes having m=n are characterized as “armchairchirality” carbon nanotubes. Illustrative metallic carbon nanotubechiralities include, for example, (3,0), (6,0), (9,0), (12,0), (15,0),(4,1), (7,1), (10,1), (13,1), (5,2), (8,2), (11,2), (14,2), (6,3),(9,3), (12,3), (7,4), (10,4), (13,4), (8,5), (11,5), (9,6), (12,6),(10,7), and (11,8).

The carbon nanotubes employed in the present disclosure can be producedby any suitable technique. Suitable carbon nanotube synthetic processescan include, for example, arc methods, laser oven, chemical vapordeposition, flame synthesis, and high pressure carbon monoxide (HiPCO).The synthetic conditions of any of these techniques can be altered tochange the chirality distribution produced, particularly to favor theproduction of carbon nanotubes with a dominant carbon nanotube chiralityor type being produced. Further, the carbon nanotubes can be purified orunpurified. Illustrative carbon nanotube purification techniques caninclude removal of metal catalysts, removal or non-nanotube carbonresidue, chirality enrichment, or any combination thereof. In someembodiments, at least removal of non-nanotube carbon residue can alsotake place in conjunction with the carbon nanotube dispersion techniquesdiscussed in further detail below.

Suitable solvents for practicing the disclosure herein can include thosethat are capable of both solubilizing carbon nanotubes and polymers inacceptable quantities. In more particular embodiments, a suitablesolvent for solubilizing carbon nanotubes and polymers, as well as forbeing incorporated in the extruded filaments disclosed herein, can beo-dichlorobenzene. Not only does o-dichlorobenzene desirably solubilizeboth carbon nanotubes and polymers at acceptable levels, but therelatively high boiling point of this solvent allows it to becontrollably removed when processing the polymer composites and extrudedfilaments disclosed herein. In o-dichlorobenzene, the practicalsolubility limit of carbon nanotubes is about 4 mg/mL, above whichviscosity begins to increase and may hamper further processing into apolymer composite of the present disclosure. A concentration of about 50g/L represents a reasonable solubility limit for the polymer. Forcertain high-performance polymers, heating may be needed to promotedissolution, and the relatively high boiling point of o-dichlorobenzenecan be advantageous in this regard. Moreover, once processing to formthe polymer composites and extruded filaments is completed, residualo-dichlorobenzene is readily retained therein until the polymercomposites or extruded filaments are ready for further use. In someembodiments, a co-solvent can also be present in combination witho-dichlorobenzene when practicing the disclosure herein. Co-solvents canbe used to further tailor the properties of an extruded filament, forexample. Other suitable solvents that can be used alone or incombination with o-dichlorobenzene include, for example,N,N-dimethylformamide, N-methylpyrollidone, propylene glycol, and ethyllactate.

Prior to forming an extruded filament, a solvent content of a solvatedcomposite containing a carbon nanomaterial, particularly carbonnanotubes, can range between about 10% to about 30% by weight. A solventcontent within this range can be sufficient to solubilize both thepolymer and the carbon nanomaterial and/or carbon nanotubes as well asto facilitate further processing operations. Specifically, a solventcontent in a range between about 10% to about 30% by weight can allow anextruded filament to be produced, in which an amount of residual solventis above 0% by weight and less than about 15% by weight.

As indicated above, the extruded filaments of the present disclosurecontain an amount of residual solvent, such as o-dichlorobenzene, thatis above 0% by weight to about 15% by weight. In some embodiments, theextruded filaments can contain about 5% to about 15% solvent by weight.In some instances, extruded filaments bearing a solvent content withinthis range can be less than desirable for additive manufacturingprocesses. For example, when too much residual solvent is present, itcan be difficult to drive off a sufficient amount of the residualsolvent when forming a printed object, excessive outgassing can occur,and the extruded filaments can possess excessive ovality forcompatibility with a print head. In some instances, however, a solventcontent within the foregoing range can be acceptable. In otherembodiments, the extruded filaments can possess an amount of residualsolvent that is above 0% to about 5% solvent by weight, or above 0% toabout 3% by weight. Extruded filaments with a lower solvent content canpossess more desirable properties in many instances.

The polymer defining the continuous phase of the polymer composites andextruded filaments disclosed herein is not considered to be particularlylimited, other than being a thermoplastic polymer. In general, anypolymer that can be co-dissolved with carbon nanotubes and processedaccording to the disclosure herein can be suitably used. For example, invarious embodiments, the polymer utilized in the disclosure herein caninclude, for example, polyketones, polystrenes such asacrylonitrile-butadiene-styrene copolymer, polyetheretherketones,polyamides, polyolefins such as polyethylene or polypropylene,polyesters, polyurethanes, polyacrylonitriles, polycarbonates,polyetherimines, polyethyleneimine, polyethylene terephthalate,polyvinyl chloride, copolymers thereof, mixtures thereof, and the like.In more particular embodiments, the polymer can be a polyetherimine or apolycarbonate, and in still more particular embodiments, the polymer canbe a polyetherimine. Polyetherimine polymers can be especially desirabledue to their high strength, thermal stability and radiation resistance,which can make them well suitable for aerospace applications. In someembodiments, a mixture of polymers can be present in combination withone another. In more specific embodiments, a polyetherimine or apolycarbonate can be present in combination with a secondary polymer.Suitable secondary polymers are not considered to be particularlylimited.

In further embodiments, a variety of other nanomaterials can besubstituted for carbon nanotubes in the polymer composites and extrudedfilaments of the present disclosure, or these alternative nanomaterialscan be present in combination with carbon nanotubes in some instances.Other nanomaterials that can be present include, for example, metalnanoparticles, non-metallic nanoparticles, nanodiamond, graphene, carbonblack, and the like. When used in combination with carbon nanotubes, thealternative nanomaterial(s) can further tailor the properties of thepolymer composites and extruded filaments, such as through lowering theelectrical impedance or increasing thermal conductivity.

Solution-based processing methods for producing the polymer compositesand extruded filaments are also disclosed herein. In variousembodiments, the methods can include dissolving a polymer and ananomaterial in a solvent to produce a solvated composite, reducing asolvent content of the solvated composite to within a range of about 10%to about 30% by weight to produce a partially desolvated composite,forming first particles of the partially desolvated composite, supplyingthe first particles to an extruder, and extruding a first filamentcontaining the polymer as a continuous phase and the nanomaterialhomogenously mixed throughout the continuous phase. At least a portionof the solvent remains in the first filament.

Alternately, the solvated composite containing a dissolved polymer and ananomaterial in a solvent can be foamed. Suitable foaming techniqueswill be understood by persons having ordinary skill in the art.

In more specific embodiments, the nanomaterial includes carbon nanotubesor a mixture of carbon nanotubes with nanomaterials such as, forexample, nanowires, nanoparticles, nanodiamond, graphene, carbon black,or the like. These alternative nanomaterials can also be employedsingularly or in combination with one another without carbon nanotubesbeing present in some embodiments.

In more particular embodiments, an amount of solvent remaining in thefirst filament can range between about 5% to about 15% solvent byweight. As indicated above, extruded filaments having a solvent contentwithin this range can be unsuitable for downstream applications, such asadditive manufacturing processes, in some instances. In more particularembodiments, the solvent content of the first filament can range betweenabout 5% to about 10%, or between about 10% to about 15%, or betweenabout 5% to about 7.5%, or between about 7.5% to about 10%, or betweenabout 10% to about 12.5%, or between about 12.5% to about 15%, orbetween about 8% to about 12% by weight.

In the event that the solvent content of the first filament isundesirably high or the first filament has one or more unwantedproperties (e.g., excessive ovality), the methods of the presentdisclosure can further include forming a second filament with a lowersolvent content. More specifically, in various embodiments, methods ofthe present disclosure can include forming second particles from thefirst filament, supplying the second particles to an extruder, andextruding the second particles to form a second filament, where thesecond filament has above 0% to about 5% solvent by weight. In moreparticular embodiments, an amount of solvent in the second filament canrange from above 0% to about 3% solvent by weight, or above 0% to about2% solvent by weight, or above 0% to about 1% solvent by weight. In morespecific embodiments, an amount of solvent in the second filament canrange from about 1% to about 2% or about 1% to about 3% solvent byweight.

In various embodiments, an amount of the nanomaterial, such as carbonnanotubes or an alternative nanomaterial, in the first filament or thesecond filament can range between about 0.0001% to about 10% by weight,or about 0.001 to about 8% by weight, or about 0.01% to about 5% byweight. Carbon nanotube loadings within this range are sufficient forconveying electrical conductivity to the extruded filament and printedobjects obtained therefrom.

When carbon nanotubes are used in producing the first filament, thecarbon nanotubes can be dispersed (i.e., exfoliated or debundled) fromone another prior to being dissolved in the solvent with the polymer.Prior dispersion of the carbon nanotubes can promote their dissolutionin the solvent and facilitate formation of the solvated composite.Suitable dispersion techniques for use in conjunction with formingextruded filaments and other methods of the present disclosure arediscussed in further detail below. Desirably, the carbon nanotubedispersion methods described herein are accomplished without utilizingsurfactants, which might otherwise become incorporated in an extrudedfilament and compromise its properties. Further, dispersed carbonnanotubes produced in accordance with the present disclosure can beisolated as a solid material without additional functionalization of thecarbon nanotubes, which would otherwise compromise the electricalconductivity of the extruded filaments and polymer composites disclosedherein.

In more specific embodiments, dispersing the carbon nanotubes from oneanother can include: dissolving a quantity of carbon nanotubes inchlorosulfonic acid under an inert atmosphere; after dissolving thequantity of carbon nanotubes in the chlorosulfonic acid, reacting thechlorosulfonic acid with an alcohol and precipitating dispersed carbonnanotubes, and collecting the dispersed carbon nanotubes. Suitabletechniques for collecting the dispersed carbon nanotubes can include,for example, any combination of washing, filtration, decantation,centrifugation, and the like. Such dispersion techniques can be employedin conjunction with the filament extrusion methods disclosed herein, orthey can be employed independently in other applications wherein it isdesired to obtain or utilize dispersed carbon nanotubes, for example,when depositing carbon nanotube layers within a nanoelectronic device.

Chlorosulfonic acid has been recognized for several years as a“universal” solvent for solubilizing and dispersing carbon nanotubesfrom one another. However, the high reactivity and acidity of thissolvent can be problematic. In conventional approaches, the solvent canundergo a reaction with moisture to produce hydrogen chloride andsulfuric acid, the latter of which can induce a functionalizationreaction of the carbon nanotubes. As indicated above, suchfunctionalization of the carbon nanotubes can be undesirable in manyinstances due to the decreased electrical conductivity that results.

As such, the inventors discovered that dispersed carbon nanotubeswithout functionalization can be produced through excluding moisturefrom the chlorosulfonic acid solution of carbon nanotubes (i.e., bymaintaining an inert atmosphere such as dry nitrogen or argon) and thencarefully reacting the chlorosulfonic acid with an alcohol such asmethanol, ethanol, propanol, isopropanol, butanol, or isobutanol, whichdoes not produce sulfuric acid. In the case of methanol, the reactionwith chlorosulfonic acid produces methyl sulfate (i.e., the monoester ofsulfuric acid), which does not undergo a reaction with the carbonnanotubes. Since the carbon nanotubes become insoluble once the methylsulfate has been formed, the carbon nanotubes can be easily isolated asdiscussed above. As such, dispersed carbon nanotubes can be readilyintroduced in the filament extrusion processes disclosed herein.

Downstream applications of the extruded filaments are also contemplatedin various embodiments of the present disclosure, as discussedhereinafter.

In some embodiments, additive manufacturing processes of the presentdisclosure can include supplying an extruded filament from a print head,and depositing the extruded filament in a desired shape in alayer-by-layer deposition process. Suitable shapes are not considered tobe particularly limited, and a suitable deposition pattern can beexecuted under computer control. In general, a printed object of anyshape that can be conventionally printed during additive manufacturingprocesses can be similarly produced using the extruded filamentsdisclosed herein. In more specific embodiments, the extruded filamentsemployed in the additive manufacturing processes can contain carbonnanotubes as a nanomaterial and a polyetherimine or a polycarbonate as apolymer. As such, printed objects of the present disclosure can includea polyetherimine or a polycarbonate as a continuous phase and carbonnanotubes mixed homogenously therein. Alternately, multiple filamentshaving a different loading of carbon nanotubes therein can be employedto provide heterogeneous carbon nanotube distributions in the printedobject, such as a gradient distribution, for example.

In some embodiments, the additive manufacturing processes of the presentdisclosure can further include an annealing operation to completeremoval of solvent in forming a final part. Heating can take place usingradiant heat, microwave radiation, and/or infrared heating, and in someembodiments, heating can take place under inert atmosphere or reducedpressure to promote solvent removal.

EXAMPLES Example 1: Production of a Carbon Nanotube Composite Powder

Single-walled carbon nanotubes (Thomas Swann Elicarb) and polyetherimide(ULTEM™) were mixed in separate portions of hot o-dichlorobenzene anddissolved. The solutions were then combined and stirred for several daysto ensure homogeneity. The resulting solvated composite (1 wt. % carbonnanotubes and polyetherimide) was placed in a vacuum oven and heated toreduce the solvent content. The vaporized o-dichlorobenzene was trappedfor subsequent recycling. After partial removal of the solvent, thesolvent content was about 30% by weight. The partially desolvatedcomposite was then cooled to −70° C. (liquid N₂) and pulverized in amill to produce small particles, which were then fed to an extruder forfurther processing.

Example 2: Dispersion of Carbon Nanotubes

Prior to dissolution in o-dichlorobenzene, the carbon nanotubes ofExample 1 were dissolved in chlorosulfonic acid under an inertatmosphere at a concentration below 5 mg/mL. Upon forming thechlorosulfonic acid solution, methanol was added to completely convertthe chlorosulfonic acid into methyl sulfate, which resulted inprecipitation of the carbon nanotubes. The mixture was filtered, and thecarbon nanotubes were rinsed with additional methanol. After rinsingwith methanol, the carbon nanotubes were further rinsed with a smallquantity of o-dichlorobenzene in preparation for dissolution in thissolvent.

The dispersed carbon nanotubes were combined in o-dichlorobenzene at aconcentration exceeding 2 mg/mL and sonicated lightly to minimize damageto the carbon nanotube structure. The solution was then centrifuged at11,000 rpm, which resulted in formation of a gel-like dispersion ofcarbon nanotubes at the bottom of the vial. The supernatant at the topof the vial, which contained tube fragments and non-nanotube carbon, wasdecanted and filtered to recover the o-dichlorobenzene for reuse. Thegel-like dispersion of carbon nanotubes contained 2-3% carbon nanotubesby weight and was added directly to the solution of polyetherimine inthis solvent in Example 1. Alternately, the gel-like dispersion can bestored for later use.

Example 3: Extrusion of a Composite Filament

The powder produced in Example 1 was fed to an extruder operating at200° C. The resulting filament had modest ovality and minimal bubbling.The solvent content of the extruded filament at this juncture was about10-15% by weight.

To improve ovality and decrease the solvent content further, thefilament was re-milled to produce particles, and the particles wereagain fed to an extruder. Extrusion of the filament took place this timeat 300° C. The resulting filament had improved ovality, and the solventcontent after the second extrusion was approximately 3% by weight.

Example 4: Characterization

FIG. 1 shows an illustrative SEM image of the extruded filament producedin Example 3. As shown in FIG. 1, the carbon nanotubes were welldispersed in the polymer matrix. FIG. 2 shows an illustrative TGAprofile of the extruded filament produced in Example 3. As shown in FIG.2, the glass transition temperature was below approximately 400° C., andgood thermal stability could be realized up to 500° C.

Various o-dichlorobenzene solutions containing a range of carbonnanotube loading were also deposited as thin films onto a silicon waferfor further electrical characterization. Table 1 summarizes the surfaceresistivity of the thin films after solvent removal as a function of thecarbon nanotube loading.

TABLE 1 Carbon Nanotube Resistivity Loading (wt. %) (Ohm-cm) 0.1 1200000.45 45 1 5.5 5 0.00945

Although the disclosure has been described with reference to thedisclosed embodiments, those skilled in the art will readily appreciatethat these only illustrative of the disclosure. It should be understoodthat various modifications can be made without departing from the spiritof the disclosure. The disclosure can be modified to incorporate anynumber of variations, alterations, substitutions or equivalentarrangements not heretofore described, but which are commensurate withthe spirit and scope of the disclosure. Additionally, while variousembodiments of the disclosure have been described, it is to beunderstood that aspects of the disclosure may include only some of thedescribed embodiments. Accordingly, the disclosure is not to be seen aslimited by the foregoing description.

What is claimed is the following:
 1. A method of forming an extrudedfilament, the method comprising: dissolving a polymer and a nanomaterialin a solvent, thereby producing a solvated composite; reducing thesolvent content of the solvated composite to within a range of about 10%to about 30% by weight, thereby producing a partially desolvatedcomposite; forming first particles of the partially desolvatedcomposite; supplying the first particles to an extruder; and extrudingthe first particles to form a first filament comprising the polymer as acontinuous phase and the nanomaterial homogeneously mixed throughout thecontinuous phase; wherein the residual amount of the solvent remainingin the first filament is above 0% by weight to 15% by weight.
 2. Themethod of claim 1, wherein the nanomaterial comprises carbon nanotubes.3. The method of claim 2, further comprising: dispersing the carbonnanotubes prior to producing the solvated composite.
 4. The method ofclaim 3, wherein the carbon nanotubes are non-functionalized afterdispersing.
 5. The method of claim 3, wherein the step of dispersing thecarbon nanotubes comprises: dissolving a quantity of carbon nanotubes inchlorosulfonic acid under an inert atmosphere; after dissolving thequantity of carbon nanotubes in the chlorosulfonic acid, reacting thechlorosulfonic acid with an alcohol and precipitating dispersed carbonnanotubes; and collecting the dispersed carbon nanotubes.
 6. The methodof claim 2, wherein the polymer comprises a polyetherimide or apolycarbonate.
 7. The method of claim 2, wherein the solvent compriseso-dichlorobenzene.
 8. The method of claim 2, wherein the first filamentcomprises about 5% to about 15% solvent by weight.
 9. The method ofclaim 8, further comprising: forming second particles from the firstfilament; supplying the second particles to the extruder; and extrudingthe second particles to form a second filament, the second filamentcomprising above 0% to about 5% solvent by weight.
 10. The method ofclaim 9, wherein the second filament comprises about 0.01% to about 5%carbon nanotubes by weight.